I want to select e.g the 80% best attributes according to the information gain. The first inner operator of the WrapperXValidation operator must return AttributeWeights, so how can I modify my AttributeWeights returned from InfoGainWeighting that the weights of the 80% worst attributes are set to zero?

since the info gain weighting would deliver always the same weights I do not see why to use the Wrapper validation for that. You could simply apply the operator AttributeWeightSelection for using the top k attributes. Here is an example: